For example, an infrared high output semiconductor laser having a wavelength of 780 nm is configured in a structure shown in FIG. 6. Specifically, on a semiconductor substrate 21 made of an n-type GaAs are laminated in order a lower clad layer 22 formed of, for example, an n-type AlGaAs based compound semiconductor; an active layer 23 formed of a non-doped, n-type or p-type AlGaAs based compound semiconductor; and an upper first clad layer 24 formed of a p-type AlGaAs based compound semiconductor. Furthermore, on the resultant lamination are laminated an etching stop layer 25 made of, for example, a p-type InGaP; an upper second clad layer 26 formed of a p-type AlGaAs based compound semiconductor; and a cap layer 27 made of GaAs. The resultant lamination is etched by wet etching, thereby forming a ridge 28.
Thereafter, a current block layer 29 made of, for example, an n-type AlGaAs based compound semiconductor is formed on both sides of the ridge 28 by selective growth. Thus, all layers from the lower clad layer 22 to the cap layer 27 and the current block layer 29 constitute a light emitting layer forming section 31, on which a contact layer 30 made of, for example, p-type GaAs is then laminated. A p-side electrode 32 is disposed on the contact layer 30; in contrast, an n-side electrode 33 is disposed on the reverse of the substrate 21.
With this structure, the semiconductor substrate 21 is normally formed in a thickness of about 50 μm, and further, the contact layer 30 is normally formed in a thickness of about 1 μm to about 3 μm. This semiconductor laser is mounted in a face down (i.e., junction down) structure, in which the p-side electrode 32 serving as an upper electrode is bonded to a sub mount or the like, thereby facilitating dissipation of heat generated in the light emitting layer forming section 31. As a consequence, the contact layer 30 is formed in as small a thickness as about 1 μm to about 3 μm, as described above. Moreover, in order to obtain a high output of 60 mW or more, a near field pattern need be enlarged. The thickness of the light emitting layer forming section 31 including the upper and lower clad layers 22, 24 and 26 and the active layer 23 should be in the range of about 4 μm to about 6 μm, so as to enlarge a beam spot P to be formed at the light emitting layer forming section 31.
In addition, as disclosed in Japanese Patent Application Laid-Open No. 2003-86886, a distance from the active layer to the reverse surface of the semiconductor substrate is made to be substantially equal to a thickness from the active layer to the upper surface of the contact layer, and as a consequence, wherein the thickness of the contact layer is set to, for example, about 50 μm and, more particularly, to 60 μm or less. In this manner, a reflection quantity due to a return light beam is reduced when a semiconductor laser is used for an optical pick-up, and it is possible to prevent degradation of an S/N of a reading signal by detecting the return light beam by a light receiving element even if the return light beam reaches an optical disk again.
However, as described above, the semiconductor laser for a high output by increasing the thickness of the light emitting layer forming section has raised problems that a catastrophically optical damage (hereinafter, referred to as “a COD”) level is low, operation for a long period of time (an aging test at high temperatures) often induces breakage, a lifetime is short, and reliability is low.